Multi-antenna wireless transmission technique, or Multiple Input Multiple Output (MIMO), can achieve spatial multiplex gain and spatial diversity gain by deploying a plurality of antennas at both the transmitter and the receiver and utilizing the spatial resources in wireless transmission. Researches on information theory have shown that the capacity of a MIMO system grows linearly with the minimum of the number of transmitting antennas and the number of receiving antennas. FIG. 1 shows a schematic diagram of a MIMO system. As shown in FIG. 1, a plurality of antennas at the transmitter and a plurality of antennas at each of the receivers constitute a multi-antenna wireless channel containing spatial domain information. Further, Orthogonal Frequency Division Multiplexing (OFDM) technique has a strong anti-fading capability and high frequency utilization and is thus suitable for high speed data transmission in a multi-path and fading environment. The MIMO-OFDM technique, in which MIMO and OFDM are combined, has become a core technique for a new generation of mobile communication.
For instance, the 3rd Generation Partnership Project (3GPP) organization is an international organization in mobile communication field and plays an important role in standardization of 3G cellular communication technologies. Since the second half of the year 2004, the 3GPP organization has initiated a so-called Long Term Evolution (LTE) project for designing Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN). The MIMO-OFDM technique is employed in the downlink of the LTE system. In a conference held in Shenzhen, China in April 2008, the 3GPP organization started a discussion on the standardization of 4G cellular communication systems (currently referred to as LTE-A systems). In this conference, a concept known as “multi-antenna multi-BS coordination” gets extensive attention and support. Its core idea is that multiple BSs can provide communication services for one or more UEs simultaneously, so as to improve data transmission rate for a UE located at the edge of a cell.
With regard to the multi-antenna multi-BS coordination, fundamental agreements are mainly available from the following standard document by March, 2010: 3GPP TR 36.814 V9.0.0 (2010-03), “Further advancements for E-UTRA physical layer aspects (Release 9)”, which can be outlined as follows:                In a multi-antenna multi-BS service, a UE needs to report channel state/statistical information of a link between the UE and each BS/cell in a set of cells. This set of cells is referred to as a measurement set for multi-antenna multi-BS transmission.        The set of BSs/cells for which the UE actually perform information feedback can be a subset of the measurement set and is referred to as a coordination set for multi-antenna multi-BS transmission. Here, the coordination set for multi-antenna multi-BS transmission can be the same as the measurement set for multi-antenna multi-BS transmission.        A BS/cell in the coordination set for multi-antenna multi-BS transmission participates in Physical Downlink Shared Channel (PDSCH) transmission for the UE, either directly or indirectly.        The scheme in which multiple BSs directly participate in coordination transmission is referred to as Joint Processing (JP). The JP scheme needs to share PDSCH signal of the UE among the multiple BSs participating the coordination and can be divided into two approaches. One is referred to as Joint Transmission (JT) in which the multiples BSs transmit their PDSCH signals to the UE simultaneously. The other one is referred to as Dynamic Cell Selection (DCS) in which at any time instance, only one of the BSs which has the strongest signal link is selected to transmit its PDSCH signal to the UE.        The scheme in which multiple BSs indirectly participate in coordination transmission is referred to as Coordinated Beamforming/Coordinated Scheduling (CB/CS). In this CB/CS scheme, instead of sharing PDSCH signal of the UE among the multiple BSs participating in the coordination, the beams/resources for transmission of PDSCHs for different UEs are coordinated among the multiple BSs to suppress the interference between each other.        For a UE operating in the multi-antenna multi-BS coordinated transmission environment, information feedback is mainly carried out separately for each BS and is transmitted over the uplink resources of the serving BS.        
As used herein, the term “information feedback” refers to a process in which a UE needs to feed back CSI to a BS such that the BS can perform corresponding operations such as radio resource management. There are primarily the following three CSI feedback approaches in the prior art documents.
Complete CSI Feedback:
The UE quantizes all elements in a transceiver channel matrix and feeds back each of the elements to the BS. Alternatively, the UE can analogously modulate all elements in the transceiver channel matrix and feeds back them to the BS. Alternatively, the UE can obtain a transient covariance matrix for the transceiver channel matrix, quantizes all elements in the covariance matrix and feeds back each of the elements to the BS. Thus, the BS can reconstruct an accurate channel from the channel quantization information fed back from the UE. This approach is described in non-patent document 1: 3GPP RI-093720, “CoMP email summary”, Qualcomm and its implementation is illustrated in FIG. 2.
Statistic-Based CSI Feedback:
The UE applies a statistical process on a transceiver channel matrix, e.g., calculating a covariance matrix thereof, quantizes the statistical information and then feeds back it to the BS. Thus, the BS can obtain statistical state information of the channel based on the feedback from the UE. This approach is described in non-patent document 1: 3GPP RI-093720, “CoMP email summary”, Qualcomm and its implementation is illustrated in FIG. 3.
CSI Feedback Based on Codebook Space Search:
A finite set of CSI is predefined by the UE and the BS (i.e., codebook space, common codebook spaces including channel rank and/or pre-coding matrix and/or channel quality indication, etc.). Upon detection of a transceiver channel matrix, the UE searches in the codebook space for an element best matching the CSI of the current channel matrix and feeds back the index of the element to the BS. Thus, the BS looks up the predefined codebook space based on the index to obtain rough CSI. This approach is described in non-patent document 2: 3GPP, RI-083546, “Per-cell precoding methods for downlink joint processing CoMP”, ETRI, and its implementation is illustrated in FIG. 4.
Among the above three approaches, the complete CSI feedback has the best performance, but is impractical to be applied to actual system due to the highest feedback overhead. In particular, in the multi-antenna multi-BS coordination system, its feedback overhead grows in proportional to the increase of the number of BSs and it is even more impractical. The CSI feedback based on codebook space search has the lowest feedback overhead, but is worst in terms of performance since it cannot accurately describe the channel state such that the transmitter cannot make full use of channel characteristics and cannot the perform transmission accordingly. However, it is extremely simple to implement and can typically accomplish feedback with a few bits. Hence, it is widely applied in actual systems. The statistic-based CSI feedback achieves a good tradeoff between these two approaches. When the channel state has significant statistical information, this approach can accurately describe the channel state with a relatively small amount of feedback, thereby achieving a relatively ideal performance.
Currently, in the LTE and the LTE-A systems, in consideration of factors for practical system implementation, the CSI feedback based on codebook space search is employed in a single cell transmission mode. In the multi-BS/cell coordination mode in the LTE-A system, it is expected that this CSI feedback based on codebook space search will continue to be used.
For the CSI feedback based on codebook space search, there are two feedback channels in the LTE system, a Physical Uplink Control CHannel (PUCCH) and a Physical Uplink Shared CHannel (PUSCH). In general, the PUCCH is configured for transmission of synchronized, basic CSI with low payload; while PUSCH is configured for transmission of bursty, extended CSI with high payload. For the PUCCH, a complete CSI is composed of different feedback contents which are transmitted in different sub-frames. For the PUSCH, on the other hand, a complete CSI is transmitted within one sub-frame. Such design principles remain applicable in the LET-A system.
The feedback contents can be divided into three categories: Channel Quality Index (CQI), Pre-coding Matrix Index (PMI) and Rand Index (RI), all of which are bit quantized feedbacks. The CQI typically corresponds to a transmission format having a packet error rate no more than 0.1.
In the LTE system, the following eight types of MIMO transmission approaches for downlink data are defined:
1) Single antenna transmission. This is used for signal transmission at a single antenna BS. This approach is a special instance of MIMO system and can only transmit a single layer of data.
2) Transmission diversity. In a MIMO system, diversity effects of time and/or frequency can be utilized to transmit signals, so as to improve the reception quality of the signals. This approach can only transmit a single layer of data.
3) Open-loop space division multiplexing. This is a space division multiplexing without the need for PMI feedback from UE.
4) Closed-loop space division multiplexing. This is a space division multiplexing in which PMI feedback from UE is required.
5) Multi-user MIMO. There are multiple UEs simultaneously participating in the downlink communication of the MIMO system.
6) Closed-loop single layer pre-coding. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is required.
7) Beam forming transmission. The beam forming technique is employed in the MIMO system. A dedicated reference signal is used for data demodulation at UE. Only one single layer of data is transmitted using the MIMO system. The PMI feedback from UE is not required.
8) Two-layer beam forming transmission. The UE can be configured to feed back PMI and RI, or not to feed back PMI and RI.
In the LTE-A system, the above eight types of transmission approaches may be retained and/or canceled, and/or a new transmission approach, dynamic MIMO switching, can be added, by which the BS can dynamically adjust the MIMO mode in which the UE operates.
In order to support the above MIMO transmission approaches, a variety of CSI feedback modes are defined in the LTE system. Each MIMO transmission approach corresponds to a number of CSI feedback modes, as detailed in the following.
There are four CSI feedback modes for the PUCCH, Mode 1-0, Mode 1-1, Mode 2-0 and Mode 2-1. These modes are combination of four types of feedbacks, including:                Type 1: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band. The respective overheads are L bits for the sub-band location, 4 bits for the CQI of the first codeword and 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword.        Type 2: wideband CQI and PMI. The respective overheads are 4 bits for the CQI of the first codeword, 3 bits for the CQI of the possible second codeword which is differentially coded with respect to the CQI of the first codeword and 1, 2 or 4 bits for PMI depending on the antenna configuration at BS.        Type 3: RI. The overhead for RI is 1 bit for two antennas, or 2 bits for four antennas, depending on the antenna configuration at BS.        Type 4: wideband CQI. The overhead is constantly 4 bits.        
The UE feeds back different information to the BS in correspondence with the above different types.
The Mode 1-0 is a combination of Type 3 and Type 4. That is, the feedbacks of Type 3 and Type 4 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-0, the wideband CQI of the first codeword in the Set S and possibly the RI information are fed back.
The Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets. In the Mode 1-1, the wideband PMI of the Set S, the wideband CQIs for the individual codeword and possibly the RI information are fed back.
The Mode 2-0 is a combination of Type 3, Type 4 and Type 1. That is, the feedbacks of Type 3, Type 4 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-0, the wideband CQI of the first codeword in the Set S, possibly the RI information as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
The Mode 2-1 is a combination of Type 3, Type 2 and Type 1. That is, the feedbacks of Type 3, Type 2 and Type 1 are carried out at different periods and/or with different sub-frame offsets. In the Mode 2-1, the wideband PMI of the Set S, the wideband CQIs for the individual codeword and possibly the RI information, as well as one preferred sub-band location in the BP and the CQI for the sub-band are fed back.
There are thus the following correspondences between the MIMO transmission approaches and the CSI feedback modes:
MIMO transmission approach 1): Mode 1-0 and Mode 2-0;
MIMO transmission approach 2): Mode 1-0 and Mode 2-0;
MIMO transmission approach 3): Mode 1-0 and Mode 2-0;
MIMO transmission approach 4): Mode 1-1 and Mode 2-1;
MIMO transmission approach 5): Mode 1-1 and Mode 2-1;
MIMO transmission approach 6): Mode 1-1 and Mode 2-1;
MIMO transmission approach 7): Mode 1-0 and Mode 2-0;
MIMO transmission approach 8): Mode 1-1 and Mode 2-1, with PMI/RI feedback from UE; or
Mode 1-0 and Mode 2-0, without PMI/RI feedback from UE.
Still, CQI, PMI and RI are primary feedback contents in the single BS transmission approach of the LTE-A system. In order that the feedback modes for a UE are consistent with those corresponding to the transmission approaches 4) and 5) while supporting a new transmission approach 9), the Mode 1-1 and Mode 2-1 in the LTE-A system are optimized for a scenario where a BS is equipped with 8 transmission antennas. That is, a PMI is collectively determined from two channel pre-coding matrix indices, W1 and W2, where W1 represents wideband/long-term channel characteristics and W2 represents sub-band/short-term channel characteristics. For transmission of W1 and W2 over PUCCH, Mode 1-1 can be sub-divided into two sub-modes: Sub-Mode 1 of Mode 1-1 and Sub-Mode 2 of Mode 1-1; the original Mode 2-1 is also modified.
In order to support the newly defined feedback mode, the following feedback types is newly introduced in the LTE-A system:                Type 1a: one preferred sub-band location in a Band Part (BP, which is a subset of the Set S and has its size dependent on the size of the Set S) and a CQI for the sub-band, plus a W2 for another sub-band. The overhead for the sub-band location is L bits. The total overhead for the CQI and the W2 is 8 bits when RI=1, 9 bits when 1<RI<5, and 7 bits when RI>4.        Type 2a: W1. The overhead for W1 is 4 bits when RI<3, 2 bits when 2<RI<8, and 0 bit when RI=8.        Type 2b: wideband W2 and wideband CQI. The total overhead of the wideband W2 and the wideband CQI is 8 bits when RI=1, 11 bits when 1<RI<4, 10 bits when RI=4, and 7 bits when RI>4.        Type 2c: wideband CQI, W1 and wideband W2. The total overhead of the wideband CQI, the W1 and the wideband W2 is 8 bits when RI=1, 11 bits when 1<RI<4, 9 bits when RI=4, and 7 bits when RI>4. It is to be noted that, in order to control feedback overhead, the value range of the W1 and the wideband W2 here is obtained by down-sampling the full value range of the W1 and the wideband W2.        Type 5: RI and W1. The total overhead for the RI and the W1 is 4 bits for 8 antennas and 2-layer data multiplexing and 5 bits for 8 antennas and 4 or 8-layer data multiplexing. It is to be noted that, in order to control feedback overhead, the value range of the W1 here is obtained by down-sampling the full value range of the W1.        Type 6: RI and PTI. PTI stands for Pre-coding Type Indicator and has an overhead of 1 bit for representing information on pre-coding type. The total overhead for the RI and the PTI is 2 bits for 8 antennas and 2-layer data multiplexing, 3 bits for 8 antennas and 4-layer data multiplexing, and 4 bits for 8 antennas and 8-layer data multiplexing.        
In this context, “W2” refer to “sub-band W2”, while “wideband W2” will be referred to in their full expressions.
The Sub-Mode 1 of Mode 1-1, Sub-Mode 2 of Mode 1-1 and the new Mode 2-1 have the following relationship with the existing types and these new types:                The Sub-Mode 1 of Mode 1-1 is a combination of Type 5 and Type 2b. That is, the feedbacks of Type 5 and Type 2b are carried out at different periods and/or with different sub-frame offsets.        The Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2/2c.                    For transmission approach 4) or 8), the Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2. That is, the feedbacks of Type 3 and Type 2 are carried out at different periods and/or with different sub-frame offsets.            For transmission approach 9), the Sub-Mode 2 of Mode 1-1 is a combination of Type 3 and Type 2c. That is, the feedbacks of Type 3 and Type 2c are carried out at different periods and/or with different sub-frame offsets.                        The new Mode 2-1 relates to transmission approach 9) only and is a combination of Type 6, Type 2b and Type 2a/1a.                    When the PTI of Type 6 is 0, the new Mode 2-1 is a combination of Type 6, Type 2b and Type 2a. That is, the feedbacks of Type 6, Type 2b and Type 2a are carried out at different periods and/or with different sub-frame offsets.            When the PTI of Type 6 is 1, the new Mode 2-1 is a combination of Type 6, Type 2b and Type 1a. That is, the feedbacks of Type 6, Type 2b and Type 1a are carried out at different periods and/or with different sub-frame offsets.                        
It is also to be noted that, in the multi-antenna multi-BS coordination, the JT and the CB/CS have different requirements on CSI. Specifically, the JT focuses on obtaining Phase Information (PI) between BSs such that an information coherence addition gain can be obtained. On the other hand, the CB/CS focuses on obtaining PMI information of a adjacent BS (including W1 and W2, W1 and W2 constituting the PMI information of a adjacent BS being referred to as enhanced W1 and enhanced W1, or eW1 and eW2, respectively), so as to effectively coordinate beams between the BSs, thereby eliminating interference. The JT and the CB/CS have different application scenarios. For example, if the background connection between BSs is reliable, the JT is applicable; otherwise the CB/CS is applicable. As another example, if a UE can accurately estimate the CSI of an adjacent BS, the JT is applicable; otherwise the CB/CS is applicable. As a further example, the closer a UE is located to the edge area of a cell, the more the JT is applicable; otherwise the CB/CS is applicable. Thus, if the UE can dynamically switch between the JT and the CB/CS based on the CSI, the data rate and/or the communication quality of the UE can be improved. The UE can apply any of the following switching criteria: calculating the data rate achievable by each of the JT and the CB/CS and selecting the approach achieving higher data rate; or calculating the bit error rate achievable by each of the JT and the CB/CS and selecting the approach achieving lower bit error rate; or any other criteria.
In summary, there are currently few references available for the CSI feedback for multi-antenna multi-BS coordination in the LTE-A system, as this has not been discussed in the standardization process. At present, the general concept is that the feedback contents involve CSI based on codebook space search, such as CQI, PMI and RI, and the information feedback is mainly carried out separately to each BS. In this architecture, there are still a number of issues to be researched. In particular, it is an important research topic on how to feed back CSI in the multi-BS coordination environment so as to be flexibly suitable for the JT and CB/CS transmissions.